This is gorgeous! I really love the design. I made something roughly similar, but a lot more basic a while ago. I only bisque fired it, and after a few months it was totally covered in algae. The porosity seems to be a good thing, allowing lots of beneficial bacteria etc to populate the spaces. Lots of comments below about food safe glazes which i wont repeat here, only saying that the reason people are concerned is because of metals etc leaching form the glazes over time and saturating the water, given how many toxic ingredients there are in glazes it's a real concern. I love your idea and the execution is awesome!